In recent years, in the field of cars, security robots, etc., a request to perform obstacle detection in a moving direction with sufficient precision for the purpose of collision prevention has increased. As a method of obstacle detection, a laser radar being a distance measuring device using light beam scanning has been known. A common laser radar is a device which measures a distance to an obstacle based on time after having projected a laser beam until receiving reflected light rays.
In the case of projecting a light flux without narrowing it, an amount of light rays which collide against an object becomes smaller, which is not suitable for measurement on a long distance. Accordingly, scanning is performed with a narrowed light flux, whereby enlargement of a detection range can be attained. As a general scanning technique, a technique to rotate or oscillate a mirror or a polygon mirror with multiple mirror surfaces has been known.
In particular, since a laser radar needs to detect a wide range, the laser radar is required to project a laser light flux to a wide range. Although a light projecting range in a scanning angle direction is determined based on a scanning angle and a spot size, since the scanning angle direction of a laser light flux becomes the rotation direction of a mirror, it is not necessary for the spot size to be made larger in the scanning angle direction. Further, in order to increase resolving power, it is preferable to make a spot size shorter in the scanning angle direction. On the other hand, a light projecting range of a sub scanning angle direction orthogonal to a scanning angle direction is determined based on the number of scanning lines and a projected light spot size (or a view field of a light receiving lens). Since there is a limitation in increasing the number of scanning lines, a projected light spot size at the center of a scanning angle has be made longer in a sub scanning angle direction. Therefore, in many cases, a spot size in a horizontal direction is different from a spot size in a vertical direction.
Furthermore, in order to perform highly precise detection, a skill is required for suppressing a change in resoling power for measurement between the center of scanning and a periphery of the scanning. As a factor to cause a change in resolving power, there are longitudinal distortion and spot rotation. Description is given to them. In FIG. 1 showing schematically a laser radar, it is assumed that a mirror unit MU includes a reflecting surface RM1 inclining relative to a rotation axis RO and the mirror unit MU is rotated around the rotation axis RO. Herein, it is further assumed that a spot light flux SL emitted from a light source LD of a light projecting system LP in a direction along the rotation axis RO has an aspect ratio other than 1.0. Accordingly, in FIG. 1, a spot light flux SL (its cross section is indicated with hatching) reflected on a reflecting surface RM1 in a measurement range proceeds in a direction perpendicular to the sheet surface of FIG. 1. At this time, its cross section is shaped in a rectangular cross section in which a length “a” in a scanning angle direction (a lateral direction in the drawing) is smaller than a length “b” (>a) in a sub scanning angle direction (a vertical direction in the drawing).
On the other hand, as shown in FIG. 2, in the case where the mirror unit MU rotates by about 30 degrees, a light flux LB reflected on a reflecting surface RM1 moves from a position shown in FIG. 1 to a lateral direction. With this movement, although scanning is performed for a range where an object exists, spot rotation arises in the spot light flux SL. Further, in the case where a light flux LB enters the reflecting surface RM in a direction not parallel to the rotation axis, longitudinal distortion also arises (spot rotation differs from the case of parallel incidence). In concrete terms, in longitudinal distortion, a spot light flux SL distorts in a sub scanning angle direction. Accordingly, an interval between scanning lines becomes narrower, or an interval becomes wider. In FIG. 2, it means a phenomenon that a spot light flux SL shifts from an originally-proceeding direction (a solid line) to an axis line direction of the rotation axis RO (illustrated with a broken line). An amount of this shift is represented by an angle deviation (⋅) in a sub scanning angle direction. On the other hand, in the case where there is a difference between the scanning angle direction and the sub scanning angle direction in a cross sectional shape of a spot light flux SL, an interval between spot light fluxes becomes narrower, or an interval becomes wider by the rotation of a spot light flux. In FIG. 2, it means a phenomenon that a spot light flux SL rotates as shown with a one-dot chain line relative to an original attitude direction (a solid line). Its rotation angle ⋅ is called spot rotation angle.
FIG. 3 is a diagram showing a relationship between a scanning angle and a spot rotation angle in the case where a light flux is made to enter a reflecting surface inclining by 45 degrees relative to the rotation axis in a direction parallel to a rotation axis. Here, in the case where a light flux is made to enter a reflective surface RM in a direction parallel to a rotation axis, a scanning angle become the same with a rotation angle. As shown in FIG. 3, as a rotation angle ⋅ of a reflecting surface RM1 increases, a spot rotation angle ⋅ increases.